A fluctuation-dissipation theorem relates \(D_s\) 
to the vector current correlator \(\langle J_i(\vec{p}=0, t+\tau ) \;J_i(\vec{p}=0, t) \rangle \)  [1]}. This correlator can be
easily calculated on the lattice. From this, an extraction of \(D_s\) 
has been attempted [2]}. This extraction though is quite
difficult: \(D_s\)  is related to the width of the transport peak of the
spectral function of the correlator. The spectral function is
connected to the correlator by an integral transform similar to a
Laplace transform. It is very difficult to invert the transform accurately
enough to extract
the width of its transport peak. The transport peak is in the infrared
and it further gets affected by the infrared cutoffs put in the
calculation, like size of the system.
